This invention relates to directly heated oxide cathodes and more particularly to surface treatment of a base of a cathode which is coated with an oxide of an alkaline earth metal.
Generally, cathodes are used with receiving tubes, discharge tubes and cathode-ray tubes. A cathode used with a cathode-ray tube should meet the requirement that it acts quickly to instantaneously show a television picture on a screen. That is, a cathode is required to have a short starting time.
There are two types of cathode. One is of an indirectly heated type and the other is of a directly heated type. An indirectly heated cathode has a starting time of about 20 seconds, but a directly heated cathode is very short in its starting time which is 1 to 2 seconds. Owing to this feature, a directly heated cathode is best suited for use as a quick action cathode.
FIG. 1 shows the essential portions, in an enlarged view, of a directly heated cathode of the prior art. In the figure, 1 designates a base to which an electric current is passed to generate heat. The base 1 is formed of a nickel-base alloy comprising tungsten for increasing mechanical strength and magnesium, zirconium, silicon and aluminum serving as reducing agents, with the amount of tungsten usually being 20 to 30% by weight.
2 designates an oxide of an alkaline earth metal with which the base 1 is coated. Generally, in coating the base 1 with the oxide 2, a carbonate of an alkaline earth metal is applied to the base 1 and then the carbonate is subjected to decomposition by heating in a vacuum into an oxide of an alkaline earth metal. When the carbonate is subjected to decomposition by heating, reactions indicated by the following chemical formulae (1), (2), (3) and (4) take place. The following description will refer to an example in which BaCO.sub.3 is used as a carbonate and the base 1 is formed of a nickel base alloy containing tungsten. EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2 .uparw. (1) EQU 3BaCO.sub.3 +W.fwdarw.Ba.sub.3 WO.sub.6 +3CO.uparw. (2) EQU 3BaO+WO.sub.3 .fwdarw.Ba.sub.3 WO.sub.6 .uparw. (3) EQU 6BaO+W.fwdarw.Ba.sub.3 WO.sub.6 +3Ba.uparw. (4)
As is clear in formula (1), BaCO.sub.3 is decomposed into the oxide 2 in the form of BaO and CO.sub.2, so that it is possible to obtain an electron emitting oxide. However, as is clear in formulae (2), (3) and (4), a layer 5 of Ba.sub.3 WO.sub.6 is formed and interposed between the base 1 and the oxide 2 as shown in FIG. 2. Because of the presence of the layer 5, operation of the cathode for a prolonged period of time has resulted in the peeling-off of the oxide 2 from the base 1. Thus the cathode of the prior art has had the disadvantage of being short in service life.
In order to eliminate the defect of the cathode shown in FIG. 1, proposals have been made to use a cathode shown in FIG. 3. In FIG. 3, 1 designates a base formed of a nickel-base alloy containing tungsten as is the case with the cathode shown in FIG. 1, and 2 designates a coating of an oxide of an alkaline earth metal, emitting electrons, which is applied to the base 1. An upper surface of the base 1 is coarsened by a powder alloy 3, mainly consisting of nickel and having a particle size of several .mu.m, which is sprayed onto the upper surface in an amount of several mg per 1 cm.sup.2 of the upper surface and fired in a vacuum or hydrogen atmosphere. The oxide 2 is rigidly secured to the base 1 through the medium of the powder alloy 3. In a directly heated cathode of the aforesaid construction, the velocity of diffusion of nickel alloy in powder form through the base 1 increases as heat is generated in the base 1, with the result that the surface of the base 1 which is in contact with the powder nickel alloy 3 is distorted and the base 1 is deformed into a spherical shape. As shown in FIG. 4, the inverse number of a curvature R of the deformation becomes greater as the period of time in which the cathode is placed in service becomes longer. In the diagram shown in FIG. 4, the abscissa indicates the time and the ordinate represents the inverse number of the curvature R showing the amount of deformation. A curve A is obtained when the base has a temperature of 950.degree. C.; a curve B, when the base has a temperature of 1000.degree. C.; and a curve C, when the base has a temperature of 1050.degree. C. The deformation of the base 1 causes a gradual change in the spacing between the oxide 2 and a grid, resulting in changes in cut-off voltage. FIG. 5 shows changes of cut-off voltage, .DELTA.Eco, during operation of the cathode shown in FIG. 3. Thus, if this cathode is used to provide an electron gun structure for a color television cathode-ray tube, the changes would cause the changes of operating point of the three electron guns, so that the electron guns would have the disadvantage that white balance is lost. An additional disadvantage is that, with an increase in the deformation of the base 1, the oxide 2 would peel off the base 1.
As aforesaid, a directly heated cathode of the prior art has the disadvantage that the formation of Ba.sub.3 WO.sub.6 tends to cause the oxide to be peeled off from the base. If a powder of nickel is deposited on the base by firing as shown in FIG. 3 in order to prevent the peeling off of the oxide from the base, the cathode would undergo a deformation and the movement of the operating point of the electron gun structure would occur, with the result that the cathode is unable to perform its function satisfactorily. Thus the present invention has been developed for the purpose of providing a directly heated cathode which obviates the aforementioned disadvantages of the prior art and which can be put to practical use with satisfactory results.